Novichok, the military-grade toxin worrying England

The Soviet-designed nerve agent Novichok which has sickened a British couple four months after it was used in a failed plot to murder a Russian ex-spy, is a military-grade poison that is poorly known and hard to trace.

Experts believe it to be far more potent than more familiar nerve toxins such as VX or sarin.

- 'Newcomer' -

The newest class of nerve agent, the Novichok family of toxins was developed by the Soviet government towards the end of the Cold War. It has never been used in warfare.

The name is Russian for "newbie" or "newcomer".

Novichok toxins are thought to be used in the form of an ultra-fine powder, or like other nerve agents, in liquid or vapour form.

"These Novichok agents are thought to consist of two separate non-toxic components that when mixed, form a nerve agent," said Michelle Carlin, a forensic chemistry lecturer at Northumbria University.

Novichok is "more dangerous and sophisticated" than sarin or VX, "and is harder to identify," added Gary Stephens, a pharmacology expert from the University of Reading.

"One of the main reasons these agents are developed is because their component parts are not on the banned list. It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier," he said.

- How does it work? -

Nerve agents are the most toxic known chemical warfare agents. They affect the central nervous system by disrupting communication between the brain, the main organs and muscles.

A signalling overload results in loss of muscle control which can lead to breathing difficulties and paralysis. Victims may asphyxiate or suffer heart failure.

- The treatment -

There is no specific antidote.

Typically, victims of nerve agent poisoning are kept on heart and lung machines while given the drug atropine.

Nerve agents attack the enzyme that controls acetylcholine, leading to an overproduction and muscle malfunction.

With time, the body clears out the nerve agent and starts producing the acetylcholine-controlling enzyme anew.

"It is possible that the pair will be being kept sedated for some time to prevent overactivity of nerves in the brain, to prevent seizures and other damage," said Alastair Hay, an environmental toxicology expert with the University of Leeds.

- Prognosis? -

The couple, Dawn Sturgess, 44, and Charlie Rowley, 45, are being looked after "in the only place with experience of successfully treating patients exposed to these agents," said Chris Morris of Newcastle University's Medical Toxicology Centre.

They are in the same hospital that treated former Russian double agent Sergei Skripal and his daughter Yulia, attacked with Novichok in March. Both survived.

This means "there's every reason to believe that the outcome for both will be good."

With rapid and intensive care, the victims stand a "reasonable chance" of recovery, said Andrea Sella, a professor of inorganic chemistry at University College London.

"But it is worth bearing in mind that victims of this kind of organophosphate poisoning who recover from the acute effects are likely to suffer neurological damage which may well be life-changing."

- Risk to others? -

Experts speculate the dose that poisoned the pair may have lingered since the Skripal attack in March.

Novichok agents are "designed to be quite persistent -- they hang around in the environment, neither evaporating or decomposing quickly," Sella said via the Science Media Centre in London.

"That means that if a container or a surface was contaminated with this material it would remain a danger for a long time and it will be vital to trace the movements of this couple to identify where they might have come into contact with the source."

The public at large is "at very low risk" from this material, Sella stressed, but "until the source is found there is a remote chance that someone else might come into contact with it."

If others had made contact with the agent, added Hay, they would already have experienced symptoms.

"In the absence of these, they should feel reassured about the places they visit routinely."